1. Field of the Invention
The present invention relates to boundary acoustic wave devices used as, for example, resonators or band-pass filters, and particularly, to a boundary acoustic wave device including a piezoelectric body, an electrode defined by a metal embedded in the upper surface of the piezoelectric body, and a dielectric body extending over the piezoelectric body and the electrode.
2. Description of the Related Art
Duplexers (DPXs) and RF filters used in mobile communication systems need to have both broad band-pass properties and good temperature properties. Conventional boundary acoustic wave devices used for such DPXs or RF filters include a piezoelectric substrate made of 36-50 degrees rotated Y-plate X-propagation LiTaO3. The piezoelectric substrate has a temperature coefficient of frequency of −45 ppm/° C. to −35 ppm/° C. A known technique for improving temperature properties is to arrange a SiO2 layer having a positive temperature coefficient of frequency over IDT electrodes arranged on the piezoelectric substrate.
In a structure in which the SiO2 layer extends over the IDT electrodes, unevenness occurs between fingers of the IDT electrodes and spaces between the electrode fingers. That is, a surface of the SiO2 layer cannot be prevented from having differences in height between the IDT electrodes and spaces therebetween. Therefore, there is a problem in that surface irregularities of the SiO2 layer cause a deterioration of insertion loss.
An increase in the thickness of the IDT electrodes necessarily increases the height of the irregularities. Therefore, the thickness of the IDT electrodes cannot be increased.
Recently, boundary acoustic wave devices have been replacing surface acoustic wave devices and have been attracting significant attention because the boundary acoustic wave devices are useful in manufacturing small-size packages. “RF Filter Using Boundary Acoustic Wave” (Proc. Symp. Ultrason. Electron., Vol. 26, pp. 25-26 (2005/11)) discloses a boundary acoustic wave device including a LiNbO3 substrate, IDT electrodes, and a SiO2 layer defining a dielectric body laminated in that order. The IDT electrodes have a relatively large thickness such that the acoustic velocity of an SH-type boundary acoustic wave propagating between the LiNbO3 substrate and the SiO2 layer is less than that of a slow transverse wave propagating in the SiO2 layer. Thus, the SH-type boundary acoustic wave is non-leaky. FIG. 3 of “RF Filter Using Boundary Acoustic Wave” (Proc. Symp. Ultrason. Electron., Vol. 26, pp. 25-26 (2005/11)) shows that the thickness of an IDT electrode that is sufficient for an SH-type boundary acoustic wave to be non-leaky is at least 0.15λ when the IDT electrode is made of Al, or at least 0.04λ when the IDT electrode is made of one of Cu, Ag, and Au, wherein λ represents the wavelength of the SH-type boundary acoustic wave.
When boundary acoustic wave devices, as well as the boundary acoustic wave device disclosed in “RF Filter Using Boundary Acoustic Wave” (Proc. Symp. Ultrason. Electron., Vol. 26, pp. 25-26 (2005/11)), include IDT electrodes which are made of Au and which have a thickness of at least 0.04λ, frequency properties of the boundary acoustic wave devices vary significantly due to differences in thickness between the electrodes. Therefore, it has been difficult to manufacture boundary acoustic wave devices having good frequency properties with high reproducibility.